大般涅槃經疏 1.36

1.36

初、「二月」下，聲時表法者，二月是仲春之時。仲，中也，即表中道。十五日是月滿之時，滿表圓常。故以仲時滿月之日，表於中道圓明之法。

First, from 'the second month' onward, that the time of the voice represents the Dharma: the second month is the time of mid-spring. 'Mid' means middle, which represents the middle way. The fifteenth day is the time of the full moon; fullness represents perfect permanence. Therefore, the day of the full moon at the mid-season represents the perfectly clear Dharma of the middle way.